Method and apparatus for controlling electron beam current

ABSTRACT

Methods and apparatus for independent control of electron emission current and x-ray energy in x-ray tubes are provided. The independent control can be accomplished by adjusting the distance between the cathode and anode. The independent control can also be accomplished by adjusting the temperature of the cathode. The independent control can also be accomplished by optical excitation of the cathode. The cathode can include field emissive materials such as carbon nanotubes.

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/679,303, entitled “X-Ray Generating Mechanism Using ElectronField Emission Cathode,” filed on Oct. 6, 2000, the entire contents ofwhich are herein expressly incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] At least some aspects of this invention were made with Governmentsupport under the sponsorship of the Office of Naval Research, contractno. N00014-98-1-0597. The Government may have certain rights in thisinvention.

BACKGROUND OF THE INVENTION

[0003] In the description that follows, reference is made to certainstructures and/or methods. However, the following references should notbe construed as an admission that these structures and/or methodsconstitute prior art. Applicants expressly reserve the right todemonstrate that such structures and/or methods do not qualify as priorart against the present invention.

[0004] X-rays occupy that portion of the electromagnetic spectrumbetween approximately 10⁻⁸ and 10⁻¹² m. Atoms emit x-rays through twoseparate processes when bombarded with energetic electrons.

[0005] In the first process, high-speed electrons are decelerated asthey pass through matter. If an individual electron is abruptlydecelerated, but not necessarily stopped, when passing through or nearthe nuclear field of a target atom, the electron will lose some of itsenergy which, through Plank's law, will be emitted as an x-ray photon.An electron may experience several such decelerations before it isfinally stopped, emitting x-ray photons of widely different energies andwavelengths. This process produces the bulk of x-ray radiation andresults in a continuous-type spectrum, also called Bremsstrahlung.

[0006] In the second process, an incident electron collides with andejects an orbital electron of a target atom. If the ejected electron isfrom an inner shell orbit, then an electron in an outer shell orbit willfall to the inner vacant orbit with an attendant emission of an x-rayphoton. In this process energy is emitted in the form of an x-ray whoseenergy or wavelength represents the orbital transition involved. Becausethe energies of orbital electrons are quantized, the x-ray photonsemitted are also quantized and can only have discrete wavelengthscharacteristic of the atom. This gives rise to their classification ascharacteristic x-rays.

[0007] Several methods have been used to produce the incident electronsat a cathode and accelerate them into a target anode. One traditionalapproach has been the use of an x-ray tube. Depending upon the methodused in generating the electrons, x-ray tubes may be classified in twogeneral groups, gas tubes and high-vacuum tubes.

[0008]FIG. 1 shows a conventional gas x-ray tube. The x-ray generatingdevice 110 is substantially made of a glass envelope 120 into which isdisposed a cathode 125 which produces a beam of electrons 140 whichstrike an anode 130 thereby causing x-rays to be emitted 150 which canbe used for sundry purposes including medical and scientific. Thecathode is powered by a high voltage power supply via electrical leads135. In addition, a gas pressure regulator 115 regulates the gaspressure in this type of x-ray device.

[0009] High vacuum tubes, an example of which is shown in FIG. 2, are asecond type of x-ray tube. FIG. 2 shows a vacuum x-ray tube device witha thermionic cathode. In this type of device 210 a glass envelope 220serves as the vacuum body. The cathode 225 is deposed within this vacuumand is provided with electrical leads 235. Electrons 240 are emitted bythermionic emission from the cathode 225 and strike an anode target 230The efficiency of such emission of x-rays is very low causing the anodeto be heated. To increase the lifetime of this device, it has beennecessary to provide a cooling mechanism. One embodiment of a coolingmechanism is a chamber 260 through which water is circulated by the useof an inlet 265 and an outlet 270. To improve the efficiency of theemitted beam of electrons a focusing shield 245 is often utilized. Thefocusing shield 245 collimates the thermionically emitted electrons anddirects them to the anode 230. However, the thermionic origin of theelectrons makes focusing to a small spot size difficult. This, in part,limits the resolution of modern x-ray imaging (see, for example,Radiologic Science For Technologist, S. C. Bushong, Mosby-Year Book,1997). X-rays 250 emitted from the anode 230 pass through a window 255and are subsequently available for sundry purposes, including medicaland scientific. An additional feature of this type of device is anexterior shutter 275. It has been found necessary to incorporate such ashutter to prevent the incidental emission of x-rays associated with theheating decay of the cathode. This is because even though theapplication of power to the cathode may be terminated, residual heatingmay be such that electrons continue to be emitted towards the target andcontinue to produce x-rays.

[0010] This process of x-ray generation is not very efficient sinceabout 98 percent of the kinetic energy of the electron stream isconverted upon impact with the anode into thermal energy. Thus, thefocus spot temperature can be very high if the electron current is highor continuous exposure is required. In order to avoid damage to theanode it is essential to remove this heat as rapidly as possible. Thiscan be done by introducing a rotating anode structure.

[0011] As noted above, a shutter (e.g. 275) is necessary in such devicesbecause thermionic emission of electrons from a cathode does not allowfor precise step function initiation and termination of the resultingelectron beam. Indeed, while still at elevated temperatures andsubsequent to removal of power, a thermionic cathode may emit electronswhich may cause unwanted x-ray emission from the target. In operationthe shutter is held open either mechanically or by means of amicroswitch.

[0012] Moreover, due to high temperature heating, the cathode filamenthas a limited lifetime, typically around a few hundred hours in medicalapplications and thousand hours in analytical applications. Under normalusage, the principle factor determining the lifetime of the x-ray tubeis often damage to the cathode filament.

[0013] The amount of useful x-rays generated in the anode isproportional to the electron beam current striking at the anode. Inthermionic emission, the electron beam current is only a small fractionof the current passing through the cathode filament (typically {fraction(1/20)}). In modern medical applications such as digital radiography andComputed Tomography (CT), very high x-ray intensity is requiredconcomitantly requiring a very high thermionic emission cathode current.Therefore, a principle limitation in these applications is the amount ofelectron beam current generated by the cathode.

[0014] A possible improvement in the generation of x-rays is theintroduction of field emission cathode materials. Field emission is theemission of electrons under the influence of a strong electric field.However, the incorporation of conventional field emission cathodematerials into x-ray generating devices presents certain challenges. Forinstance, the field emission cathode materials must be capable ofgenerating an emitted electron current density of a sufficiently highlevel (can be as high as 2000 mA on the target for medical applications)such that, upon striking the anode target material, the desired x-rayintensity is produced.

[0015] Many conventional field emission materials are incapable ofproducing the desired emitted electron circuit density absent theapplication of a relatively high electrical field to the cathode.Moreover, many of the conventional field emission materials cannotproduce stable emissions at high current densities under high appliedelectrical fields. The use of high control voltages increases thelikelihood of damaging the cathode material, and requires the use ofhigh powered devices which are costly to procure and operate.

[0016] In many applications, such as in medical diagnostics andtreatments, it is desirable to have independent controls on the electronemission current (mA) and the x-ray energy (kVp). However,conventionally the emission current from a field emission cathode iscontrolled by varying the voltage between the cathode and anode in adiode structure, or in the case of a triode structure, between theelectron emission surface and the gate structure in the cathode. Varyingthe voltage will change the electron kinetic energy bombarding the x-raygenerating target materials. Since the x-ray energy is determined by theelectron kinetic energy, the resulting x-ray will have differentenergies. However, these conventional methods do not allow forindependent controls of the electron emission current and the x-rayenergy. Accordingly, it would be desirable to independently control theelectron emission current and the x-ray energy.

SUMMARY OF THE INVENTION

[0017] The present invention provides methods and apparatus forcontrolling electron beam current. Specifically, the present inventionallows independent control of the electron emission current and thex-ray energy. In accordance with the exemplary embodiments of thepresent invention, the cathode is coated with electron field emissionmaterials, such as carbon nanotubes. Moreover, in exemplary embodimentsof the present invention, the cathode and anode are disposed in a vacuumtube.

[0018] In accordance with a first embodiment of the present invention,independent control of the electron emission current and the x-rayenergy is achieved by adjusting the distance between the cathode andanode. In accordance with one aspect of this embodiment, the distance isadjusted by varying an amount of electricity applied to a piezoelectricmaterial which is attached to the cathode and/or the anode. Inaccordance with another aspect of this embodiment, the cathode and/oranode is mounted on a mechanically adjustable platform.

[0019] In accordance with a second embodiment of the present invention,independent control of the electron emission current and the x-rayenergy is achieved by adjusting the temperature of the cathode.

[0020] In accordance with a third embodiment of the present invention,independent control of the electron emission current and the x-rayenergy is achieved by shining an optical light source on the cathode.Specifically, the optical light source, for example a photon source,emits protons on the field emissive material of the cathode to adjustthe carrier density and temperature of the field emissive materials. Inaccordance with one aspect of this embodiment, the cathode comprisesmultiple emitters which are selectively bombarded by the optical lightsource.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0021] Objects and advantages of the invention will become apparent fromthe following detailed description of preferred embodiments thereof inconnection with accompanying drawings in which like numerals designatelike elements and in which:

[0022]FIG. 1 is a cross-section of a conventional gas x-ray tube;

[0023]FIG. 2 is a cross-section of a conventional vacuum x-ray tube;

[0024]FIGS. 3a-3 d illustrate an exemplary x-ray tube in accordance witha first embodiment of the present invention;

[0025]FIGS. 4a and 4 b illustrate an exemplary x-ray tube in accordancewith a second embodiment of the present invention;

[0026]FIG. 5 illustrates the measured electron emission current as afunction of temperature under two different applied electric fields inaccordance with exemplary embodiments of the present invention; and

[0027]FIGS. 6a-6 c illustrate an exemplary x-ray tube in accordance witha third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] In the following description, for the purposes of explanation andnot limitation, specific details are set forth in order to provide athorough understanding of the present invention. However, it will beapparent to one skilled in the art that the present invention may bepracticed in other embodiments that depart from these specific details.In other instances, detailed descriptions of well know methods, devices,and circuits are omitted so as not to obscure the description of thepresent invention.

[0029] Exemplary embodiments of the present invention allow forindependent control of electron emission current and x-ray energy infield emission cold cathode x-ray tubes. Exemplary field emission coldcathode x-ray tubes are described in U.S. patent application Ser. No.09/679,303 entitled “X-ray Generating Mechanism Using Electron FieldEmission Cathode.” This patent application describes a field emissionnano structure cathode material for use in an x-ray generating device.The nano structure field emission material is capable of producing, in acontrolled and reliable manner, a high emitted electron current densitythrough the application of a relatively small control electrical field.Accordingly, a substantially higher electron beam current can beachieved compared with that of thermionic emission. The nano structurefield emission cathode is capable of providing precise step-functioninitiation and termination of the emission of electrons in a pulsevarying duration simply by varying the applied voltage. Using the x-raytubes with nano structure based field-emission cathodes of thisapplication, it is possible to construct portable x-ray machines for usein the field. For more information regarding these structures, theinterested reader should refer to International Publication No.02/31857A1, which corresponds to the '303 application, the entiredisclosure of which is herein expressly incorporated by reference.Although the following describes the use of such field emission coldcathode x-ray tubes in connection with the present invention, oneskilled in the art will recognize that methods and apparatus describedherein are equally applicable to other types of structures.

[0030]FIGS. 3a through 3 d illustrate an x-ray tube in accordance with afirst embodiment of the present invention. In accordance with the firstembodiment of the present invention, the electron emission current iscontrolled by adjusting the distance between the anode and the cathode.FIG. 3a illustrates a cathode 305 comprising a substrate 302 and a fieldemissive material 304. The field emission cathode 305 can be a layer of1-dimensional (1D) nano-objects such as nanotubes and nano wiresdeposited on a metal substrate 302. The 1D nano-objects can consist ofat least one of the following elements: carbon, nitrogen, boron, oxygen,Si, Ge, Ga, In, metals, carbide, nitrides, carbides, and oxides. Mountedon the field emissive material 304 is a focus ring 306 for focusing theemissions from the field emissive materials 304 onto anode 308. Anode308 is mounted on support 310. Underneath the substrate 302 is apiezoelectric material 312. The piezoelectric material 312 receiveselectrical energy from controller 314. The electrical connection betweencontroller 314 (located outside the x-ray tube) and the piezoelectricmaterial 312 (inside the x-ray tube) is made through the use of a vacuumfeed-through. One of ordinary skill in the art recognizes how to makeand use a vacuum feed-through, and hence, a detailed description of thisstructure is omitted.

[0031] In accordance with the first embodiment of the present invention,a voltage V_(a) is established between cathode 305 and anode 308. For agiven voltage V and a distance D between the anode and the cathode, theapplied electric field, i.e., the x-ray energy, is E=V/D. In electronfield emission the electron emission current I is related to the appliedelectric field E through the Fowler-Nordheim equation as follows:

J=aE ² exp(−bφ ^(3/2) /E)

[0032] where J is the emitted electron current, φ is the work functionof the field emission materials, E is the applied electric field, and aand b are Fowler-Nordheim parameters which are dependent upon theparticular setup geometry and nanostructures of the emissive materials.One of ordinary skill in the art knowing the particular setup geometryand the nanostructure of the emissive materials can readily determine aand b.

[0033] Accordingly, it can be seen that the electric field E isproportional to the voltage applied between the cathode and anode(V_(a)), and inversely proportional to the distance (D) between them.

[0034] In accordance with the first embodiment of the present invention,the relative position between the anode and the cathode are selected tocorrespond to a distance which will produce the most frequently usedelectron beam current in the most commonly used x-ray emission energyfor a particular application setting of the x-ray tube. The voltageV_(a) is selected to produce the desired x-ray radiation, and thelocation of the cathode is adjusted to achieve the desired electron beamcurrent. Accordingly, the total emission current increases when thedistance (D) between the anode and the cathode is reduced, and decreaseswhen the distance (D) between the anode and the cathode increases. Inaccordance with exemplary embodiments of the present inventioncontroller 314 is constructed such that the operator of the x-ray devicecan set the desired electron beam current, and the device willautomatically adjust the cathode-anode distance.

[0035]FIG. 3b illustrates a second arrangement in accordance with thefirst embodiment of the present invention. In both FIGS. 3a and 3 b thex-ray generating target material is mounted on the support. In FIG. 3a,the generated x-ray radiates away from the support, whereas in FIG. 3bthe x-ray radiates through the support.

[0036]FIG. 3c illustrates a third arrangement in accordance with thefirst embodiment of the present invention. In FIG. 3c piezoelectricmaterial 312′ is mounted on anode support 310. In accordance with thisarrangement, the cathode 305 is stationary and the anode 308 is movedrelative to the cathode by the application of electricity by controller314 to piezoelectric material 312′.

[0037]FIG. 3d illustrates a fourth arrangement in accordance with thefirst embodiment of the present invention. As illustrated in FIG. 3d,piezoelectric material is mounted to the support 310 of anode 308 and tocathode 305. Accordingly, the distance between the anode 308 and cathode305 is adjusted by applying electricity to piezoelectric material 312and/or piezoelectric material 312′.

[0038] In another arrangement in accordance with the first embodiment ofthe present invention, either the anode or the cathode is mounted on atranslation stage that enables translational motion to adjust thedistance between the cathode and the anode. Translation stages beingwell known in the art, one of ordinary skill in the art will recognizehow to make and use a translation stage in connection with the presentinvention for mechanical adjustment of the distance between the cathodeand the anode.

[0039]FIGS. 4a and 4 b illustrate a second embodiment of the presentinvention. In accordance with the second embodiment of the presentinvention, the temperature of the cathode is controlled to adjust theelectron emission current. Specifically, the higher the temperature ofthe cathode 305, the higher electron emission current. Accordingly,controller 314 is connected via a vacuum electrical feed-through toelectric heater 316. By controlling heater 316, the electron emissioncurrent from cathode 305 can be controlled independently of the voltageV_(a). The heater 316 can be made of metal filaments in a ceramichousing or other similar structures. Similar to the first embodiment ofthe present invention, controller 314 is designed such that an operatorneed only enter the desired electron emission current and the controllerwill adjust the heater 316 accordingly.

[0040]FIG. 5 is a graph illustrating the electron emission current as afunction of the temperature for two different applied electrical fields.As can be seen from the curves in FIG. 5, to control the emissioncurrent, the temperature of the cathode should be increased to above500° C. More particularly, as illustrated in FIG. 5, to achievemeaningful control over the current, the cathode should be heated above400° C. Heating the cathode to lower temperatures, e.g., 100° C.,provides minimal change in the current density, and hence, provideslittle, if any, control over the current.

[0041]FIGS. 6a through 6 c illustrate the third embodiment of thepresent invention. In accordance with the third embodiment of thepresent invention, the electron emission current and x-ray energy areindependently controlled by optical excitation of the field emissionmaterials by shining an optical light source on the cathode. Inaccordance with the third embodiment of the present invention, the fieldemission cathode 304′ can be a layer of nanotubes deposit on metalsubstrate 302, wherein the nanotubes comprise at least one of thefollowing element C, N, B, and O, or a layer of nano rods. Asillustrated in FIG. 6a, controller 314″ controls photon source 616 foremitting photons on emissive material 304′. In accordance with the thirdembodiment, the controller 314 is external to the vacuum tube, whereasthe photon source 616 can be either internal or external to the vacuumtube. If photon source 616 is external to the vacuum tube, the photonsource emits photons onto the emissive material 304′ via an opticalwindow.

[0042] In accordance with the third embodiment of the present invention,the voltage V_(a) is increased to a value just below the thresholdvoltage for field emission from the cathode 305′. At this point noelectrons are emitted from the cathode, and hence, no x-ray radiation isproduced. The photon source is then turned on which causes a bombardmentof the electron emissive material by photons. The electrons in theemissive material 304′ are excited to high energy states due to theexcitement of the incident photons. The excited electrons tunnel throughthe energy barrier and field emit. The emitted electrons are acceleratedby the electrical field between the anode and the cathode 305′ andbombard on the anode (not illustrated). X-ray radiation is then emittedfrom the anode (not illustrated). Accordingly, the field emittedelectron current, and therefore the intensity of the x-ray radiationgenerated, can be varied without changing the acceleration voltageV_(a), by varying the intensity of the incident photons.

[0043]FIGS. 6b and 6 c illustrate a second aspect of the thirdembodiment of the present invention. In accordance with this aspect, thefield emission cathode 305′ can comprise multiple emitters. Again, avoltage is established between the cathode and anode such that theelectrical field is just below the threshold for emission. A focusedphoton beam is scanned across the surface of the cathode 305′. The flux,i.e., number of photons per unit area per unit time, and the energy perphoton is adjusted such that the area on the cathode that is bombardedby the photons field emit electrons whereas the rest of the cathode doesnot. This provides a convenient technique for addressable emission fromthe individual emitters or groups of emitters on the cathode surface. Byscanning the beam across the cathode surface, an array or matrix ofelectron beams can be generated from the cathode, and as a result, anarray or a matrix of x-ray beams can be produced using such a system.

[0044] The present invention has been described above in connection withx-ray radiation systems to illustrate the advantageous aspects thereof.Such a description should not be limiting on the present invention.Specifically, the present invention is equally applicable to instrumentsand devices which employ electrons other types of radiation, such asgamma ray radiation, ultraviolet ray radiation, or visible lightradiation. Such instruments and devices include, but are not limited to,field emission displays, microwave tubes, electron beam lithography,electron soldering machines, plasma ignition sparks, transmissionelectron microscope (TEM), scanning electron microscope (SEM), and otherelectron spectroscopy instruments. Moreover, those of ordinary skill inthe art will recognize that the present invention can be practiced inany type of device where electron beams are generated by field emissionwith applications of electric voltage between field emissive materials(cathodes) and targets (anodes).

[0045] The present invention has been described with reference toseveral exemplary embodiments. However, it will be readily apparent tothose skilled in the art that it is possible to embody the invention inspecific forms other than those of the exemplary embodiments describedabove. This may be done without departing from the spirit of theinvention. These exemplary embodiments are merely illustrative andshould not be considered restrictive in any way. The scope of theinvention is provided by the appended claims, rather than the precedingdescription, and all variations and equivalents which fall within therange of the claims are intended to be embraced therein.

What is claimed is:
 1. An electron beam current generating apparatuscomprising: a cathode; an anode; and a controller which independentlycontrols an emission current from the cathode and electron energystriking the anode by controlling a voltage applied between the cathodeand anode, and controlling a distance between the cathode and anode, atemperature of the cathode to a temperature above 400° Celsius, or ashining of photons on the cathode.
 2. The electron beam currentgenerating apparatus of claim 1, wherein high energy electrons bombardthe anode, thereby generating radiation from the anode.
 3. The electronbeam current generating apparatus of claim 2, wherein the radiation isgamma ray radiation, x-ray radiation, ultraviolet ray radiation, orvisible light radiation.
 4. The electron beam current generatingapparatus of claim 2, wherein radiation flux and radiation energy arecontrolled by said electron beam current and the electron energy.
 5. Theelectron beam current generating apparatus of claim 1, furthercomprising: a distance adjuster, controlled by the controller, to adjusta distance between the cathode and the anode.
 6. The electron beamcurrent generating apparatus of claim 5, wherein the distance adjustermechanically adjusts the distance.
 7. The electron beam currentgenerating apparatus of claim 5, wherein the distance adjusterelectrically adjusts the distance.
 8. The electron beam currentgenerating apparatus of claim 7, wherein the distance adjuster is apiezoelectric material.
 9. The electron beam current generatingapparatus of claim 8, wherein the piezoelectric material is connected tothe anode.
 10. The electron beam current generating apparatus of claim2, wherein the piezoelectric material is connected to the cathode. 11.The electron beam current generating apparatus of claim 2, wherein thepiezoelectric material is connected to the anode and the cathode. 12.The electron beam current generating apparatus of claim 1, furthercomprising: a temperature control element, controlled by the controller,to adjust a temperature of the cathode to a predetermined temperatureabove 400° Celsius.
 13. The electron beam current generating apparatusof claim 12, wherein the temperature control element is an electricalheater connected to the cathode.
 14. The electron beam currentgenerating apparatus of claim 1, further comprising: a photon source,controlled by the controller, to shine photons on the cathode.
 15. Anelectron beam current generating apparatus comprising: a cathode; ananode; and means for independently controlling an emission current fromthe cathode and electron energy striking the anode by controlling avoltage applied between the cathode and anode, and controlling adistance between the cathode and anode, a temperature of the cathode toa temperature above 400° Celsius, or a shining of photons on thecathode.
 16. The electron beam current generating apparatus of claim 15,wherein high energy electrons bombard the anode, thereby generatingradiation from the anode.
 17. The electron beam current generatingapparatus of claim 16, wherein the radiation is gamma ray radiation,x-ray radiation, ultraviolet ray radiation, or visible light radiation.18. The electron beam current generating apparatus of claim 16, whereinradiation flux and radiation energy are controlled by said electron beamcurrent and the electron energy.
 19. The electron beam currentgenerating apparatus of claim 15, further comprising: means foradjusting a distance between the cathode and the anode, wherein themeans for adjusting is controlled by the means for controlling.
 20. Theelectron beam current generating apparatus of claim 15, furthercomprising: means for adjusting a temperature of the cathode to apredetermined temperature above 400° Celsius, wherein the means foradjusting is controlled by the means for controlling.
 21. The electronbeam current generating apparatus of claim 15, further comprising: meansfor optical excitation of the cathode by shining of photons on thecathode, wherein the means for optical excitation is controlled by themeans for controlling.
 22. A method for generating electron beam currentcomprising: providing a cathode; providing an anode; and independentlycontrolling an emission current from the cathode and a radiation energybetween the cathode and anode by controlling a voltage applied betweenthe cathode and anode, and controlling a distance between the cathodeand anode, a temperature of the cathode to a temperature above 400°Celsius, or shining of photons on the cathode.